Not applicable.
1. Technical Field of the Invention
The present invention generally relates to methods and apparatus for recovering sulfur and hydrogen from hydrocarbon processing streams. More specifically, the present invention relates methods and apparatus for processing a mixture of hydrogen sulfide, methane and/or light alkanes and oxygen in a series of reactors to produce elemental sulfur and hydrogen.
2. Description of Related Art
Many petroleum feed streams and their separated fractions contain sulfur. Sulfur is generally undesirable in most petroleum refining products, however. Therefore, refineries typically upgrade the quality of the various petroleum fractions by removing the sulfur. Specifically, hydrodesulfurization units are used to break down the sulfur compounds in the petroleum fractions and convert the sulfur to H2S. Such hydrodesulfurization units consume hydrogen because hydrogen bonds to the removed sulfur to produce the product H2S. In addition, other reactions take place concurrently, including double bond saturation, aromatic saturation, and denitrification. All of these reactions consume hydrogen.
The sources of hydrogen in a refinery include the catalytic reformer. Purified hydrogen is also produced (as a byproduct) from coking and catalytic cracking reactions. It is often the case, however, that these sources of hydrogen are insufficient to supply the entire hydrogen requirements for the refinery. Hence, it is often necessary to provide hydrogen from an additional source. Hydrogen can be produced from steam reforming of light hydrocarbons, such as methane, and from the water gas shift of the steam reformer off gas. Less desirably, hydrogen can also be purchased from outside sources, usually as the byproduct of some chemical process.
In addition to hydrodesulfurization processes, other conversion processes in a typical refinery, such as fluid catalytic cracking, coking, visbreaking, and thermal cracking, produce H2S from sulfur containing petroleum fractions. The H2S from both the desulfurization processes and these conversion processes is typically removed from the gas streams or light liquid hydrocarbon streams using chemical solvents based on alkanolamine chemistry or physical solvents. A circulating, regenerative H2S removal system employing an absorption stage for H2S pickup and a regeneration stage for H2S rejection produces a concentrated stream of H2S.
In conventional systems, this H2S stream is then fed to some type of H2S conversion unit, which converts the H2S into a storable, saleable product such as elemental sulfur, sodium hydrosulfide solution, or sulfuric acid. Conversion of the H2S to elemental sulfur is most common, primarily because elemental sulfur is the most marketable sulfur compound of those mentioned. The process most commonly used to recover elemental sulfur from H2S gas is the modified Claus sulfur recovery process.
The modified Claus sulfur recovery process has been in use since 1883 without significant changes. The process in its current form consists of a thermal reactor followed by waste heat removal, sulfur condensation, and varying numbers (usually two or three) of reheat, catalyst bed, and sulfur condensation stages. Many of the Claus plants are followed by Claus plant xe2x80x9ctail gasxe2x80x9d treatment units which process unreacted H2S, SO2, various compounds such as COS and CS2, and elemental sulfur vapor into H2S, which is then recycled back to the thermal stage of the Claus process or converted to SO2, which is absorbed in aqueous solutions to form bisulfite salts. Other tail gas treatments entail either operating Claus catalyst beds at temperatures below the dew point of sulfur or direct oxidation of the remaining H2S to sulfur either over a bed of solid catalyst or in a liquid contacting device.
The thermal stage of a conventional Claus process is a burner in a refractory lined chamber. H2S, along with other compounds such as CO2, methane and light hydrocarbon gases, nitrogen, ammonia, and hydrogen, is fed to the burner. Air, pure oxygen, or a mixture of both is fed to the burner. A flame is used to ignite the mixture of gases. In the flame, ⅓ of the H2S is oxidized by the reaction:
H2S+3/2O2xe2x86x92SO2+H2Oxe2x80x83xe2x80x83(1)
The remaining H2S then reacts with the SO2 in the flame according to the following equation, to form elemental sulfur and water:
2H2S+SO2xe2x86x923/xSx+2H2Oxe2x80x83xe2x80x83(2)
The overall reaction is:
3H2S+3/2O2=3/nSn+3H2Oxe2x80x83xe2x80x83(3)
The Claus combustion chamber typically operates at 950xc2x0 C.-1,480xc2x0 C. and converts 50 to 70% of the sulfur contained in the feed gas into elemental sulfur, depending on the temperature. The efficiency decreases with the gas residence time in the reactor. The sulfur formed by the thermal stage is recovered as a liquid by first cooling the hot reaction gases (typically from 950 to 1480xc2x0 C.) in a firetube boiler, followed by condensation of the sulfur in the tubes of a low pressure steam generator. Removing the liquid sulfur allows the equilibrium Claus reaction (3) (above) to shift to the right, to form more sulfur.
At low temperatures (below about 260xc2x0 C.) sulfur formation via the Claus reaction is known to be 90 to 98% efficient, but requires a catalyst to achieve an acceptable reaction rate. Hence, the gas exiting the low pressure steam generator, containing the unreacted H2S and SO2 in the 2/1 ratio required for the Claus reaction, is heated to a temperature that is sufficient to initiate rapid reaction. This temperature is usually in excess of 200xc2x0 C., and above the dew point of sulfur in order to keep newly-generated sulfur from condensing in the catalyst bed. Heat for this purpose can be supplied by any suitable means. The gas passes over a catalyst and the Claus reaction resumes until equilibrium is again reached. The reactor effluent stream is cooled and sulfur is again condensed out of the gas stream. The reheat of the gases, catalytic reaction, and sulfur condensation is repeated. Typically, two to three such catalytic stages are employed.
The Claus process is universally used to convert H2S to sulfur. There have been some improvements on the process, which have been related to: burner design; more active and durable catalysts; new types of reheaters; and the use of oxygen to replace air as the oxidizer. The latter improvement has significantly increased the processing capability of the process. Nevertheless, the process has remained essentially the same since its invention.
Even though it is useful both in recovering the sulfur generated in refinery processes and in reducing sulfur emissions from refineries, the process is generally viewed as relatively costly and is performed mainly out of environmental necessity. One of the economic penalties of the Claus process is that the hydrogen used to form H2S in the upstream processes is lost by forming water in the oxidation of the H2S. In a refinery where the hydrogen-generating processes do not keep pace with the rate of hydrogen consumption and hydrogen must therefore be externally supplied, sulfur recovery using the Claus process is particularly undesirable. Hence, it would be desirable to have a process that effectively recovers sulfur from an H2S stream while returning usable hydrogen to the system.
The present invention provides a system, process and apparatus for recovering elemental sulfur from various streams containing H2S without adding to the hydrogen consumption load of a refinery. The apparatus comprises a Claus reactor in which the burner assembly is replaced with a reactant mixing device and a thin layer of reactor catalyst that is highly transparent. The catalyst bed is preferably separated from the mixing device by a radiation barrier (which also provides thermal insulation). The catalyst catalyzes the partial oxidation of H2S and methane in the presence of oxygen (air) to form elemental sulfur and synthesis gas (carbon monoxide and hydrogen). In certain preferred embodiments, the process includes a cobalt-molybdenum hydrogenation catalyst in contact with the tail gas, which causes a water shift reaction to produce hydrogen from CO and water.
According to certain embodiments of the invention, a method for treating a stream containing H2S comprises mixing the H2S-containing stream with a light hydrocarbon stream and an oxygen containing stream to form a feed stream. The method also includes contacting the feed stream with a catalyst and raising the temperature of the stream sufficiently to allow oxidation of the H2S and partial oxidation of the light hydrocarbon to produce a product stream containing elemental sulfur, CO, and hydrogen. According to the method, the product stream is then cooled sufficiently to condense at least a portion of the elemental sulfur and produce a tail gas.
In certain embodiments, the method for treating a stream containing H2S, comprises mixing the stream with a light hydrocarbon stream and an oxygen containing stream to form a feed stream. This method includes contacting the feed stream with a catalyst for less than about 10000 microseconds, and simultaneously raising the temperature of the stream sufficiently to allow oxidation of the H2S and partial oxidation of the light hydrocarbon such that a product stream containing elemental sulfur, CO, and hydrogen are produced. The method also includes cooling the product stream sufficiently to condense at least a portion of the elemental sulfur and produce a tail gas.
A system in accordance with the invention comprises employing an above-described apparatus in an above-described method. According to certain embodiments, the system includes a mixing zone, a reaction zone and a cooling zone. An H2S-containing stream is mixed with a light hydrocarbon stream and an oxygen containing stream to form a feed stream in the mixing zone. In the reaction zone the feed stream is contacted with a catalyst such that elemental sulfur is formed from the H2S and such that carbon monoxide is formed from the light hydrocarbon. In the cooling zone the elemental sulfur is condensed.